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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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  <head>
    <title>Fluorescent Whitener Additive Compensation</title>
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      charset=windows-1252">
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    <h2><u>Fluorescent Whitener Additive Compensation (FWA Compensation)</u></h2>
    <br>
    <h3>Introduction</h3>
    To make paper look "whiter" without increasing the cost of
    production, paper manufactures often employ a couple of different
    techniques. One technique is to add "shading agents" to the paper,
    that absorb a little of the middle wavelengths, thereby changing the
    color of the paper to be a little less green. By far the most
    powerful way of making the paper appear more white is to add
    Fluorescent Whitener Additive (FWA, or Optical Brightening Agents -
    OBA) to the paper. This is basically a fluorescent material that
    absorbs light at Ultra Violet (U.V.) wavelengths, and re-emits it at
    a slightly longer blue wavelengths. Subjectively something that
    appears more blue, is regarded as being "whiter".<br>
    <br>
    For more technical treatment of this topic, please refer to this
    excellent paper: &lt;<a
      href="http://www.axiphos.com/BrightnessReview.pdf">http://www.axiphos.com/BrightnessReview.pdf</a>&gt;<br>
    <br>
    <h3>Fluorescence</h3>
    Fluorescent materials absorb light radiation at one wavelength, and
    then almost instantaneously re-emit some of that energy at a longer
    wavelength. Typical FWA absorbs wavelengths in the U.V. between
    about 300 and 400 nm, and re-emit it between 400 and 460nm. The
    visual effect of FWA depends on the amount of it present in the
    paper, and the amount of U.V. in the illumination compared to the
    level of normal, visible light. Generally better quality papers have
    lower levels of whitening agents, and cheaper papers more. <br>
    <br>
    <h3>Reflection Models and Spectro-colorimetry</h3>
    The way a spectrometer measures the effect of ink on paper, depends
    on a model of how an illuminant is reflected by the ink and the
    paper. Typically a spectrometer instrument illuminates the sample
    with a known illumination, often a incandescent tungsten lamp having
    a color temperature of&nbsp; 2800 degrees Kelvin. It measures the
    amount of light reflected by the sample at each wavelength, and then
    converts that to spectral reflectance value between 0 and 100% by
    dividing by it's measurement illuminant's intensity at each
    wavelength. When it comes time to use that measurement to create an
    ICC profile, the intensity of the assumed viewing illumination at
    each wavelength (typically D50 for standard ICC profiles) is then
    multiplied by the reflectance at each wavelength, and the overall
    spectral reflectance is in this way converted into CIE tri-stimulus
    values using an observer model.<br>
    <br>
    So while the instrument measures with one type of light (type A, or
    a white LED), it returns a measurement as if it had been measured
    under a different kind of light (D50) by making use of a simple
    model of light reflection off the media.<br>
    <br>
    Notice that a key assumption of this simple model is that the light
    that impinges on the sample at a given wavelength is reflected back
    at exactly the same wavelength at a diminished intensity. Notice
    also that any sort of fluorescent material (such as FWA) breaks this
    model, since fluorescent materials emit light a different
    wavelengths to which they absorb it. So the color measurements do
    not accurately portray the appearance of the media when FWA is
    present. A more complicated bi-spectral measurement (2 dimensional
    spectral reflectance) is actually needed to fully characterize
    fluorescent materials.<br>
    <br>
    <h3>What Argyll's FWA compensation does</h3>
    The FWA compensation function in Argyll improve on this simple model
    of spectral reflection by taking into account the action of FWA. To
    do this, it needs to measure the amount and nature of the FWA in the
    media, and then have enough information about the viewing
    environment to model how that FWA will behave.<br>
    <br>
    To be able to measure the level of FWA in the media, the instrument
    needs to be able to "see" the FWA in action, so the instrument needs
    to be illuminating the samples with some level of U.V. Typically all
    instruments do this, unless they have been fitted with a filter that
    filters out any U.V. illumination (so called "UV cut" instruments),
    or use an illumination source such as a "white" LED that doesn't
    emit any U.V.<br>
    Such UV excluded instruments are not suitable for use with FWA
    compensation.<br>
    The effects of FWA are modeled spectrally, so a spectral reading
    instrument is also required.<br>
    <br>
    Argyll can compute a model for the effects of FWA given the media's
    spectral characteristics, and the illuminations spectral
    characteristic, which must include the levels of U.V. in the
    illuminant. Given these two things, Argyll can calculate how much
    effect the FWA will have on the light being reflected and emitted by
    the media under the intended illumination.<br>
    <br>
    Ideally the level of FWA would be measured by comparing the paper
    spectrum with and without U.V. present in the instruments
    illumination. Because not all instruments allow these two
    measurements to be done without some sort of manual intervention,
    Argyll avoids the need for an FWA inactive (UV cut) or extra UV (UV
    LED) measurement by employing a heuristic to estimate the FWA
    inactive spectrum from the spectrum of the paper with FWA active.
    Being a heuristic, it can sometimes be fooled by certain paper
    colors into estimating more or less FWA content than is actual
    present. The heuristic works best with high quality papers with an
    essentially flat non-FWA enhanced spectrum. Papers with colored
    tints or particularly off white appearance may not work well with
    FWA compensation, unless the instrument has the capability of
    measuring with two different levels of UV.<br>
    <br>
    <img alt="Graph showing FWA effect on UV vs. UV cut measurement."
      src="FWA_measure.jpg" style="width: 387px; height: 284px;"><br>
    <br>
    Note that typically in Argyll, if a viewing illuminant is specified,
    then it is used for computing the appearance under that illumination
    (CIE XYZ values), and if FWA compensation is used, then that same
    illuminant will be assumed for the simulated measurement illuminant.
    This results in measurements that better reflects the appearance as
    the media as if it was being viewed under that illuminant, FWA
    effects and all.<br>
    <br>
    &nbsp;It is possible to also simulate the measurement of a media
    under one illuminant, while then computing the tristimulus values as
    if being viewed under a different illuminant, but this scenario is
    only really useful for reproducing standardized measurement
    conditions such as ISO 13655:2009 M0, M1 and M2, and is less useful
    than the normal FWA compensation scenario in modelling real world
    situations.<br>
    <br>
    [The Argyll FWA compensation algorithm is described in the paper: <font
      color="#000000"><font face="Times, serif"><font style="font-size:
          8pt;" size="1"><a
            href="http://www.imaging.org/IST/store/epub.cfm?abstrid=22190">A
            Practical Approach to Measuring and Modelling Paper
            Fluorescense for Improved Colorimetric Characterisation of
            Printing Processes", <i>Graeme W. Gill, Proc. IS&amp;T/SID
              11th Color Imaging Conference</i></a><span
            style="font-style: normal;"><a
              href="http://www.imaging.org/IST/store/epub.cfm?abstrid=22190">,
              Scottsdale, Arizona; November 2003; p. 248-254</a><font
              size="1">, and <font size="1">w<font size="1">as f<font
                    size="1">irst publi<font size="1">shed <font
                        size="1">on December 2, 2002</font></font></font></font></font></font></span></font></font></font>
    in the argyllu_2002_12_02 source code. ]<br>
    <br>
    <h3>Using FWA Compensation with proofing</h3>
    The most common situation for employing FWA compensation, is in
    proofing. This is when you have one printing device, the target (say
    a printing press), and wish to emulate the behaviour of it with a
    different device, the proofer (say an inkjet printer). The aim is to
    be able to put both prints next to each other in a viewing booth,
    and have them look identical. Typically the printing process, the
    inks, and the media will be different between the target device and
    the proofer. The aim of applying color profiling is to compensate
    for these differences. Since the printing process can only darken a
    white media, the selection of the proofing stock is critical.
    Ideally it should be exactly the same color as the target, or if not
    possible, lighter, so that the proofer can tint the proofing media
    to match the target. If the two media had identical levels and types
    of FWA in them, then there would be no need to use FWA compensation,
    since the appearance of the media would match under any viewing
    condition. Typically though, the levels and types of FWA are
    different between the target paper and the proofing paper. A
    limitation imposed by tri-stimulus colorimetry is that the
    differences between the two media, inks and FWA can only be
    compensated for perfectly, under a fixed and known illuminant.<br>
    <br>
    By allowing Argyll to model the effects of FWA for both the source
    profile (the target device), and the destination profile (the
    proofing device), the effects can be accounted for, modeled
    accurately, and incorporated in the profiles, so that a subsequent
    transformation from source to destination device spaces using
    absolute colorimetric intent, achieves a (hopefully) perfect
    colorimetric reproduction. Since this is a closed system, where both
    the source and destination profiles are made for each other,
    non-standard parameters such as illuminant and observer models can
    be used, as long as they are the same for both profiles. For
    proofing, FWA should be applied identically to both profiles, by
    specifying the same illuminant, and (optionally) the same observer
    model.<br>
    <br>
    [ In practice it is possible to compensate for the color shift that
    results in viewing the media under non-D50 illumination or using a
    non 1931_2 observer, or allowing for FWA effects without severe
    incompatibility because all rendering intents except absolute
    rendering normalize to the media color, rendering the media white as
    white, even though the absolute values are not measured using a D50
    illuminant. ]<br>
    <h3>Using FWA compensation for single, general use profiles</h3>
    For creating ICC profiles that will be interchanged with other
    unknown ICC profiles, or used with non-print source or destination
    profiles, there is less flexibility, since ICC profiles by
    convention assume that all media is being viewed under D50
    illumination. The implication of this is that to be fully
    interchangeable, it's not really possible to make the profile for
    your actual viewing environment. Note that the D50 values that are
    calculated without FWA compensation do not actually reflect the
    appearance of a media under real D50, because they fail to take into
    account the different levels of FWA activity between the
    illumination using by the instrument to measure the media, and real
    D50. To allow for this and actually meet the letter of the ICC
    specifications, FWA compensation should ideally be used when
    building a interchangeable ICC profile, by selecting the D50
    illuminant, and the 1931_2 observer model (ISO 13655:2009 M1). Note
    however that this is likely to make profiles <b><span
        style="text-decoration: underline;">less</span></b>
    interchangeable rather than more, since few if any other profiles
    will represent the appearance under real D50, since few if any
    instruments use a real D50 illuminant that will trigger the correct
    level of FWA response, and few if any other packages will compensate
    for the differences in FWA activity between the instrument
    illuminant used and real D50 (ie. most instruments are actually
    returning&nbsp; ISO 13655:2009 M0 measurements).<br>
    <br>
    Similarly, the effects of viewing the media in an environment with a
    UV filter fitted over the D50 illuminant can be simulated by using
    FWA compensation with the D50M2 illuminant, and the 1931_2 observer,
    thereby simulating the results one would get if the media had been
    measured with a "UV cut" type instrument, although such profiles are
    not technically ICC compatible.<br>
    <br>
    <h3>Measuring the illuminant</h3>
    For FWA compensation to work well, it is necessary to know what the
    spectral shape of the illuminant used for viewing is. While many
    instruments provide an illuminant measurement capability over the
    visible spectrum, for FWA compensation it is desirable to know the
    Ultra Violet (UV) component of the illuminant. Few color instruments
    are capable of reading to such short wavelengths though. Argyll
    provides an indirect way of estimating the UV component of an
    illuminant using its <a href="illumread.html">illumread</a>
    utility. Using illumread in combination with FWA compensation is the
    recommended approach to modelling real world appearance of paper
    containing FWA.<br>
    <br>
    <h3>FWA myths</h3>
    Amongst the user (and to some degree) vendor community, there is a
    widely held belief that the solution to fluorescent whitener
    affecting color profiles is to simply use a UV filter fitted
    instrument. Exactly what the origin of the legend is, is hard to
    tell. Possibly it is a misinterpretation of the&nbsp; ANSI
    CGATS.5-1993 Annex B recommendations for measuring the impact of
    fluorescent effects, a translation of some of paper whiteness
    measurement standards into the color profiling world, or possibly in
    some common situations, if the viewing environment is very poor in
    UV, then adding a UV filter to the tungsten instrument illuminant
    makes for a better instrument/viewing illuminant match. There seems
    to be no scientific or practical basis for believing that a UV
    filter fitted instrument magically makes all FWA induced problems go
    away.<br>
    <br>
    <h3>Instrument UV filters</h3>
    Note that to be able to measure the FWA in the paper, the instrument
    has to be able to trigger Fluorescence, which it cannot do if it is
    fitted with a UV filter, or uses a light source that emits no UV
    (e.g. a white LED). So UV excluded instruments are not suitable for
    use with FWA compensation.<br>
    <br>
    <br>
    <br>
    <br>
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